Local area network (LAN) cabling is used to connect equipment such as personal computers, printers and fax machines that pass information between them using high-speed digital signals. This type of high performance cabling is sometimes referred to as telecommunications cable. Since an office contains many computers, computer file servers, printers, and fax machines, the LAN cabling interconnects all of this equipment into a communications network. LAN cabling has been designed to support telecommunication between all of the individual elements of the network.
FIG. 1 shows an example of LAN cabling, in a simplified drawing. FIG. 1 shows how the LAN cabling, most of which runs within the building walls, is used to connect the personal computer 1 at someone's desk to the file server 2 in the telecommunications room. The maximum length of cable 3 inside the wall cannot exceed 90 meters. Wall jack connectors 4 are used to connect the cords 5 from the computer and file server to the LAN cabling.
Cabling: Cabling is an important word in the term LAN cabling because cabling includes the connectors 4 placed on the LAN cable as well as the cable 3 itself. Thus, the performance of the LAN cabling depends upon the connectors as well as the cable.
Installation: Technicians install the LAN cabling as a part of new construction or as part of a LAN performance upgrade in existing structures. In either case, the technicians cable 3 through the walls and then place the connecting jacks 4 on the ends of the cable. The jacks are then snapped into the wall jack mounting plate and the installation is complete.
However, the technician is then required to test each LAN cabling run or link with calibrated test equipment. This testing certifies to the general contractor that the cabling run has been correctly installed from the standpoint of signal integrity. Hand-held LAN testers are used to perform these tests. The testers drive the cabling with a series of different signal types and from measurements of the received signals, determine if the cabling is capable of supporting the telecommunication signals at the prescribed data rate.
The LAN testers record the results of each test and, at a later time, print out a test document indicating that the link passed or failed. The technician gets paid for the links that pass. If there are links that fail, the technician must re-test, and often replace connectors that have been incorrectly or improperly installed. The technicians keep testing and repairing the links until they all pass.
LAN Testers: LAN testers are fairly sophisticated hand-held test systems, which can test LAN links with a series of tests covering a frequency range of 1 to 250 MHz, in the case of TIA Category 6 cabling. FIG. 2 shows a typical LAN tester 6, with a test adapter circuit board 7 connected to the LAN tester. The test adapter circuit board includes a test jack connector 8. The purpose of this test adapter is to provide a connection interface between the LAN tester and the LAN link to be tested.
The test jack 8 allows the LAN tester 6 to connect to the LAN link with a patch cord 9, as shown in FIGS. 3 and 4. Typical lengths for patch cords are two meters, or approximately six feet. This length allows the technician to conveniently connect the LAN tester to the wall jacks 4 during test runs.
Standards: Technicians test their installed links with reference to telecommunication industry standards. In the United States the standard is specified by the TIA or Telecommunications Industry Association. In Europe the standard comes from ISO, or International Standards Organization. When testing a link, the technician selects which type of link is being tested and the corresponding sets of measurement limits, whether from TIA or ISO.
The link is tested and the measured results are compared to limits from the specified standard. If no limits are exceeded, the link passes. If not, then the link fails and the technician must work on the failed link, as required, until it passes. Often this means reinstalling the connectors on the ends of the cable.
Standard Link Definitions: FIG. 5 shows the standard permanent link, in simplified form, with 90 meters of LAN cable, running within a structure's wall, or overhead in the ceiling. The wall jacks, attached to the cabling ends, are used to connect the link with equipment in the telecommunications room and to individual items such as computers or printers within the office's local area network. The TIA and ISO specify the length of 90 meters as the maximum length for the permanent link.
Link Testing: FIG. 6 illustrates how the LAN testers check the performance of a link. When testing a link (a procedure known in the industry as “shooting” a link), two LAN testers are required as shown. The technician connects a display end LAN tester 6A at one end of the link, and the remote end LAN tester 6B at the other end of the link. Since the display end LAN tester has a display screen to show the measurement test results, the technician shoots the link from the display end, controlling the test from there, and viewing the test results.
During the test, first one unit applies test signals to one end of the link while both units measure the results. Then the roles are reversed with the signal application and signal measurement taking place at the opposite ends of the link. When the test is complete, the remote unit sends its data measurement files to the display unit for final processing and storage within the display unit. The limits for each test, specified by the selected standard, are applied to the measurement data set to determine if the link passed or failed the certification test.
Standard Links: Both the TIA and ISO have defined two types of LAN links, the channel link and the permanent link. Each link is shown and discussed below.
Channel Link: The channel link includes the LAN link and the patch cords, as shown in FIG. 7, but does not include the connection to the channel test adapter boards 7A. The channel link measurement path includes the link 3 inside the walls, the mated connector pairs at the walls and the patch cords and is supposed to represent the performance of the final, complete telecommunications link, which also uses patch cords to connect the personal computers and file servers to each other. Since there is a longer length of cabling in this path, the test limits for the channel link are not as stringent as those for the permanent link.
Permanent Link: The permanent link includes the link 3, plus the mated connector pairs at the wall jack, but it does not include the patch cord, as shown in FIG. 8. Nor does it include the connection to the permanent link test adapter board 7B. The permanent link test evaluates only the cable within the walls, the connector jacks at the wall, the plugs that are inserted into the jacks, and two centimeters of cable that is attached to each of the plugs. The permanent link test essentially represents the performance of just the link cabling within the walls. Consequently, the permanent link test limits are the tightest measurement limits to pass.
As a result, technicians are often told that if their link fails the permanent link test, to change over the LAN tester limits to channel link limits and re-test. If the channel test passes, the link may then be considered to pass under these conditions.
Consideration will now be given to the test issues faced by the technicians as they test their installed Local Area Network (LAN) cabling for compliance with the appropriate TIA or ISO measurement test limits. The technician will certify the installed link to either permanent or channel link measurement limits. It is assumed that the technician has performed steps necessary to calibrate the test equipment in the field before the LAN certification test to assure maximum LAN tester measurement accuracy.
Permanent Link Testing Issues
1. Permanent Link Adapter Construction: Note the prior art permanent link test adapters 7B shown in FIG. 8. Keep in mind the permanent link comprises the cable in the wall plus the mated connector pair at the wall jacks, but it does not include most of the patch cord. The permanent link adapters (PLA's) are typically fabricated by cutting a patch cord in half, and then soldering each of the cut patch cord ends to a printed circuit board (PCB) within the permanent link test adapter housing. These PCB's are designed to cause very little signal integrity problems so that their effects are ignored.
2. Permanent Link Testing Lifetime: Permanent link adapters have a limited test lifetime due to mechanical flexing of the patch cord as it enters the PLA housing. When the patch cord has been flexed beyond its maximum number of flexures, it will require replacement. When this happens, the entire PLA has to be replaced. In addition, for maximum test accuracy, both PLA's, the one at the display end and the one at the remote end should be replaced.
3. Dedicated PLA: The LAN testers often use a dedicated PLA for each permanent link tested. This is because the circuit and transmission line properties of the patch cord can be an important part of the overall PLA measurement result. The installation technician needs to be aware of what link he or she is testing, who made the cabling, and what is the preferred type of PLA to use.
4. Matched PLA Sets: Usually the technician will use a set of PLA's matched to the cable type, by vendor, which is used in the link. If the link is made with cabling, (that is, cable plus connectors), from Vendor X, then a PLA made from Vendor X patch cords will be used for the certification test.
5. PLA Cost: The PLA's can be a costly item for the installers, often $400 or more for a set of two. If the LAN cabling installation testing company has several installers, each requiring several different sets of vendor specific PLA's, this overhead item can be rather costly. The cost comes from a dedicated printed circuit board, within a plastic housing, to form the structure of the PLA, which connects to the LAN tester.
6. PLA Cross-talk: In addition, as LAN certification moves to frequencies above 250 MHz, the performance of the PLA's as a part of the measurement system becomes more critical. The measured cross talk or lack of isolation between conductor pairs within the PLA connection circuit board becomes a serious issue as frequencies increase. When the isolation degrades beyond a certain level, the LAN tester cannot measure the cabling pair-to-pair isolation because it cannot “see” past its own PLA generated crosstalk.
The present invention provides the solution to this problem. The solution is to use a connector with proven isolation properties on the test adapter board, and then to connect to that test adapter board with a patch cord having a connector which mates to the connector on the adapter board.
7. PLA Reference Plane Calibration: The last issue with permanent link adapters is that of the measurement reference plane location. The purpose of permanent link calibration is to refer all permanent link measurements to a known point along the patch cord. In particular, the permanent link measurement reference plane is calculated to set this point at the end of the patch cord, 2 centimeters from the wall jack. From this calibration, all effects from the patch cord are removed from the permanent link measurement. The calibration procedure used to define and set this reference plane at this point can involve taking an initial set of permanent link calibration data and finally referring it to this desired reference plane.
Channel Link Testing Issues
1. Channel Link Adapters: Note the channel link test adapters 7A shown in FIG. 7. Keep in mind the channel link includes the link (i.e., the cable in the wall plus the mated connector pairs at the wall jacks) and the patch cords but it does not include either the plugs or the jacks at the channel test adapter boards. The channel link adapters (CLA's) are fabricated by placing a right-angle connector with appropriate isolation on the printed circuit board mounted within the CLA housing. The right angle connector is selected to provide significant pair-to-pair isolation when mated with the patch cord used for the channel link certification.
2. CLA Testing Lifetime: Channel link adapters have a much longer test lifetime when compared to permanent link test adapters since the use of low cost replaceable patch cords solves the patch cord mechanical flexure problem. The connector mounted on the printed circuit board inside the CLA eventually wears out as the cladding on the contacts wears off. Nevertheless, the testing lifetime for the channel link adapter is considerably longer than that for the permanent link adapter.
3. Dedicated CLA: The LAN testers also use a dedicated CLA when testing channel links since low cross talk, high isolation connectors 8 are used on the channel link adapter printed circuit board.
4. Matched CLA Sets: Matched CLA sets are used by definition by virtue of the high isolation right angle printed circuit board connectors mounted on the PCB within the CLA housing. However, when compared to the PLA, any type of patch cord can be used with the CLA, so long as the patch cord is compliant with the cabling category used for the link under test.
5. CLA Cost: The CLA's are less costly than PLA's, since they can use any compliant patch cord to connect to and test the channel link.
6. CLA Cross-talk: The channel link pair-to-pair isolation is superior to that of the permanent link by virtue of the low crosstalk connector used within the CLA module housing.
7. CLA Reference Plane Calibration: The last issue with channel link adapters is also that of the measurement reference plane location. In particular, the channel link measurement reference plane is set at the end of the patch cord connector right at the input end of the patch cord, as shown in FIG. 7. With this calibration, all effects from the patch cord input connector (i.e., the plug at the tester end) are removed from the channel link measurement.
LAN Link Measurement Issue Summary
From the preceding discussion, when compared to channel link adapters, permanent link measurements require the use of a separate set of permanent link adapters, which add an undesirable set of costs in terms of: 1) the permanent link adapters themselves; 2) the number of dedicated PLA sets; and 3) limited PLA test lifetime due to patch cord flexure failure. Permanent link adapters also have more problems with minimizing pair-to-pair crosstalk when compared to channel link adapters.